The gas detector tubes of interest at present generally consist of small-diameter glass tubes containing at least one gas-permeable substrate impregnated with a chemical reagent which will change color when exposed to at least a minimum quantity of one or more specific gases. The gas detector tubes normally are prepared in advance by the manufacturer and sealed at both ends. In use, both ends of the tube are broken off and the tube is attached to the suction side of a calibrated air pump capable of drawing a predetermined volume of air (or air-gas mixture) through the substrate and reagent. When there is a sufficient concentration of the looked-for gas present in the air drawn through the tube, the specific reagent carried by the substrate begins to discolor at the upstream boundary (starting end) of the substrate packing and continues to discolor further down the length of the tube with increasing volume of flow. If the suction pump employed is properly calibrated and properly used, the input air flow usually is discontinued before all of the reagent in the tube becomes discolored. In most applications, the length of stain produced by reaction between the looked-for gas and the tube contents becomes an approximate measure of the specific gas concentration in the air sample drawn through the tube. Depending upon the apparatus manufacturer, various methods and aids are provided to the observer to assist in measuring the length of stain in the sampling tube. Such aids may consist of calibrated marks on the surface of the glass tube, measuring scales into which the tube can be inserted or nomograph charts along which the tube can be rolled to match the length of stain to the volume of air sampled and the appropriate date of manufacture and calibration for the particular batch of tubes employed.
Since gas detector tubes of the above type and their associated pump and measuring apparatus are relatively inexpensive, their use for environmental safety and health analyses can become widespread in sampling for various undesirable gases and vapors in concentrations at or near their Threshold Limit Values.
Until now, the lack of standardization of the manner in which individual people visually judge the exact location of the end-of-stain in obtaining detector tube readings has restricted the usage of these valuable indicators. While some stain demarcations are relatively sharp, all stain endings actually consist of a gradient of gradually decreasing stain with the exact end-point often very poorly defined. Under such conditions, it is not unreasonable to expect a substantial degree of scatter in readings made by individuals in a group of human observers. Visual measurements, where judgment is required, are subject to variations in lighting, visual acuity of observer and the states of stress, fatigue, and motivation of the observer. When one adds to this the normal variations and tolerances to be expected in manufacture of the sampling tubes, it is not surprising that at least one manufacturer publishes predicted relative standard deviations (for gas detector tubes of the same batch) ranging from 15% to 25%.
Important factors other than those discussed above may tend to compound the generation of discrepancies in length of stain readings made by different people at different places. One obvious problem is the phenomenon of fading of the stain after a comparatively short period of time. For instance, in attempting to measure concentrations of chlorine or nitrogen dioxide with some gas detector tubes, significant fading of the stain can be expected within a period of as little as 15 minutes. Conversely, in using one gas detector tube to test for sulphur dioxide it is necessary to wait for 10 minutes or more for the stain to develop fully before making the reading.
Somewhat more than 100 varieties of gas detector tubes presently are commercially available in the United States for detecting and measuring concentrations of gases and vapors in the Threshold Limit Value concentration ranges. While most of the stain reactions proceed from light to dark, the inverse is the case in a substantial number. The variety of colors that must be recognized occupies virtually the entire visible spectrum. In a few instances the length of the stain is less important as a concentration measure than the intensity or change of intensity of the stain relative to a standard color reference supplied with the particular detector tube concerned. In addition to the simple, single-packing tube, gas detector tubes also are employed which contain a pre-conditioning filter layer designed to remove interfering gases before the sample enters the final stain-reagent area of the tube. In another, more complex, form of gas detector tube a breakable reagent ampule is built into the tube. In such tubes the ampule is crushed by the operator to add a liquid or gaseous conditioning reagent to the sample air stream to activate the actual measurements.